Several types of methods are known, consisting in coating the collector area of a solar absorber with a black deposit to increase the absorptivity and to reduce, where appropriate, the emissivity. One method presently used consists in coating the surface with an appropriate paint, but this method incurs several serious drawbacks. In the first place it results in a collecting surface at low selectivity or none, that is, with a high emissivity, so that a substantial part of the energy is lost by radiation, essentially within the infrared. Furthermore, these coatings degrade as a rule at moderate or high temperature (frequently starting at about 100.degree. C.) after some time of service.
Another method provides for cathode depositions of chromium oxides, denoted as "chromium black", on the collector surface. The electrolytic bath is based on chromic acid and deposition is by conventional electrolysis, so that material is added to the surface. The drawback of this kind of method is the complexity and the cost of preparing the collector surface to be coated with chromium black: the surface requires nickeling beforehand to achieve good adhesion for the chromium-black coat and satisfactory selectivity. Moreover, a drop in absorptivity beyond moderate or high temperature (200.degree. C.) is noted for surfaces coated in this manner.
Another kind of method, for instance illustrated in the French patent published as No. 2 325 002, uses stainless steel as the surface to be treated, this surface being treated chemically by immersing it while hot into complex baths containing chromium-based or lead-based compounds in order to form oxide films. This type of method suffers from the drawback of involving baths which are costly and polluting, and in resulting in oxide surface layers of which the thickness must be strictly controlled and monitored to achieve selectivity, and of which the stability is not assured at high temperatures.
Furthermore another method is known (French Pat. No. 2 337 320), consisting in forming on a metal surface a film composed solely of the metal sulfide of that surface, by initiating a surface reaction using sulfur or a sulfide; this method however results in a sulfur surface layer with poor physical-chemical stability; this is a very serious drawback considering the strenuous service conditions of the solar collectors (corrosive or humid atmosphere). Also, the optical properties obtained, in particular the selectivity, are relatively mediocre and vary in such methods.
All the above-described prior methods result in adding a deposit, a film, or a coating to the surface and adhering to it more or less firmly depending on the conditions of use.
On the other hand, the patent application No. 79 18414 in the name of the applicant describes a novel process treating the material itself to a given depth from the surface and making it possible to achieve selective absorption surfaces with excellent absorptivity and low emissivity while nevertheless enjoying the benefit of exceptional thermal stability. This process consists in performing a controlled anode oxidation of a chromium-containing iron alloy.
It is the object of the present invention to provide a novel process offering optical performance and thermal stability at least equivalent to those of the above process while also having the advantage of simplified implementation and totally or nearly totally eliminating the consumption of electric power.
Another object of the present invention is to substantially improve the physical-chemical stability of the absorption surfaces and in particular their stability in the presence of water.
Another object is to create an absorber which can operate within a range of high temperatures without thereby suffering a loss in selectivity; in particular it is the object of the invention to achieve collector surfaces suitable for use as solar collectors operating with semi-focused beams and/or operating in vacuum.